In prior art, thermoplastic elastomers based on a mixture of saturated or unsaturated monoolefin copolymer rubber (EPM or EPDM) and crystalline polyolefin resin were described and have been prepared conventionally by dynamic partial curing of the mixture of rubber and resin.
For example, in U.S. Pat. Nos. 3758643 and 3862106 elastomers were prepared by partial cross-linking EPDM with peroxides and then blending with polyolefin; and in U.S. Pat. No. 3957919 elastomers were prepared by using EPDM and PP, PE, extender oil with peroxide, by means of a Brabender mixer in a batch system.
In addition, in U.S. Pat. No. 4104210 thermoplastic elastomers were prepared by dynamically cross-linking a rubber such as polybutadiene or nitrile rubber, SBR or polyisoprene, and a polyolefin resin with a curing agent. In U.S. Pat. Nos. 4212787 and 4247652 elastomers were prepared by using an olefin rubber, which is cross-linkable to peroxide, and a polyisobutylene rubber, which is not cross-linkable to peroxide, in a Banbary mixer.
Upon specifically classifying prior techniques as above, injection molding of rubber for preparing thermoplastic elastomer was carried out at an early stage by blending additives with the rubber, injecting the blend into a mold, and then cross-linking it.
However, this process entails disadvantages in that the mass production of rubber products is difficult due to the necessity of using a special molding machine, the long injection time and the need to carry out a number of complicated steps. Thus, in order to overcome such disadvantages the use of soft plastics, such as PVC, EVA and LDPE, which can be processed without cross-linking but which have properties similar to rubbers is suggested.
However, although these soft plastics have good processability and high flexibility, they have such drawbacks as low heat-resistance and low elasticity which limit their use. In addition, in order to increase the strength and heat-resistance of soft plastics, blending them with a resin having a high melting point such as HDPE or PP, has been done. This blending, however, also has many disadvantages, for example, in the thick part of the molded product sinkmarks are apt to occur because the blending causes a loss of fluidity. Accordingly, thermoplastic elastomers having properties which fall between those of cross-linked rubbers and soft plastics, have recently been developed.
Thus, in order to improve the wear resistance, tear resistance, and impact strength of olefin resins they have been blended with a rubber material. However, rubber has a lower fluidity than olefin resin and the compatibility of rubber with olefin resin is not so high. Consequently there are problems in the occurence of sinkmarks or flowmarks. Such problems become more significant if the olefin resins are blended with cured rubber.
As specific examples, the above U.S. Pat. Nos. 3758643 and 3862106 disclosed that the first rubber is cross-linked with organic peroxide or sulfur and in the next step polyolefin resin is added thereto and blended to prepare the thermoplastic elastomer. However, this process has disadvantages in that desirable properties such as elasticity, tensile strength, stiffness, surface hardness and heat distortion temperature are reduced, and particularly the thermoplastic elastomer has little or no fluidity and accordingly in a thick or large molded product flowmarks occur, and further, since the process is carried out via a batch system, the procedures are complicated and in a mass production the quality of products may be unstable depending upon the lot.
In addition, U.S. Pat. Nos. 4212787 and 4247652 disclose that thermoplastic elastomer was prepared by blending a cross-linkable rubber (EPM or EPDM) and a non cross-linkable rubber (e.g. polyisobutylene), which is not cross-linked with a cross-linking agent, together with a polyolefin resin in a Banbury mixer and then cross-linking them. The elastomer thus prepared has good physical properties. However, since the process is carried out by blending with a Banbury mixer and then treating with the cross-linking agent, the procedures are complicated, and further, since the process is carried out in a batch system there is again the danger of irregular quality in each lot.
Accordingly, the present invention solves the problems in the above conventional techniques wherein thermoplastic elastomer is prepared in a continuous system (i.e. one stage), not depending on the conventional batch system and multi-step process, and provides excellent processability and physical properties in a novel resin composition which includes special additives.